In recent years, research has been conducted actively on organic electroluminescent elements including organic materials as hole transport materials or charge transport materials constituting electroluminescent elements. Previously, compounds, e.g., anthracene derivatives, anthraquinoline derivatives, imidazole derivatives, styryl derivatives, hydrazone derivatives, triphenylamine compounds, poly-N-vinylcarbazole, and oxadiazole, are known as such a charge transport material.
Liquid-crystal compounds have been applied as display materials to various apparatuses. For example, liquid-crystal compounds are used in watches, electronic calculators, televisions, personal computers, cellular phones, and the like. Liquid-crystal substances are classified into thermotropic liquid crystals (temperature transition type liquid crystals) and lyotropic liquid crystals (concentration transition type liquid crystals) on the basis of the means for effecting the phase transition. These liquid crystals are classified into three types, smectic liquid crystals, nematic liquid crystals, and cholesteric liquid crystals, from the view point of the molecular arrangement. The liquid crystal is also known as an anisotropic liquid, and exhibits optical anisotropy similar to that of an optically uniaxial crystal. The orthoscopic observation is the observation with common crossed Nicols, and is useful for the discrimination of the types of liquid crystal and the determination of transition temperature of a liquid-crystal phase. Each liquid crystal exhibits a characteristic birefringent optical pattern under this observation, and the smectic liquid crystal is further classified into A, B, C, D, E, F, G, and the like.
Hanna et al. have proposed charge transport materials including liquid-crystal compounds having charge transport capability, wherein a liquid-crystal phase has a smectic phase. For example, a liquid-crystal charge transport material exhibiting smectic liquid-crystal property and having a reduction potential within the range of −0.3 to −0.6 (V vs. SEC) relative to a standard reference electrode (SCE) (refer to Patent Document 1), a liquid-crystal charge transport material in which a liquid crystal compounds having a self orientation property and exhibiting a smectic phase is blended with a predetermined amount of fulleren C70 having a sensitization function (refer to Patent Document 2), a liquid-crystal charge transport material dispersion type polymer film in which a liquid-crystal compound exhibiting a smectic phase is contained in a organic polymer matrix (refer to Patent Document 3), a liquid-crystal charge transport material in which a mixture containing a smectic liquid-crystal compound is contained (refer to Patent Document 4), a liquid-crystal charge transport material exhibiting a smectic liquid-crystal property and having an electron mobility or hole mobility velocity of 1×10−5 cm2/v·s or more (refer to Patent Document 5), a liquid-crystal charge transport material containing a smectic liquid-crystal compound including a functional group capable of forming a new intermolecular or intramolecular bond and a functional group having a hole and/or electron transport property in a molecule (refer to Patent Document 6), and the like have been proposed.
With respect to the smectic liquid-crystal compound proposed as described above, the charge transport is performed in a smectic A phase liquid-crystal state by using a smectic liquid-crystal compound having a 6π-electron system aromatic ring, e.g., a benzene ring, a pyridine ring, pyrimidine ring, a pyridazine ring, a pyrazine ring, or a tropolone ring; a 10π-electron system aromatic, e.g., a naphthalene ring, an azulene ring, a benzofuran ring, an indole ring, an indazole ring, a benzothiazole ring, a benzoxazole ring, a benzimidazole ring, a quinoline ring, an isoquinoline ring, a quinazoline ring, or a quinoxaline ring; or a 14π-electron system aromatic ring, e.g., a phenantone ring or anthracene. However, the above-described charge transport method requires photoexcitation, and the conductivity thereof was 10−13 s/cm without photoexcitation and 10−11 s/cm even with photoexcitation, which were values in a region of an insulating material.
In order to solve these problems, the inventors of the present invention proposed previously distyryl based compounds having a specific structure including a smectic phase as a liquid-crystal phase, the distyryl based compounds serving as liquid-crystal compounds exhibiting excellent conductivity without photoexcitation (for example, refer to Patent Documents 8 to 10) and, furthermore, proposed organic electroluminescent elements and thin film transistors including the distyryl based compounds (refer to Patent Document 11).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 09-316442    Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-162648    Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-172118    Patent Document 4: Japanese Unexamined Patent Application Publication No. 11-199871    Patent Document 5: Japanese Unexamined Patent Application Publication No. 10-312711    Patent Document 6: Japanese Unexamined Patent Application Publication No. 11-209761    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2001-351786    Patent Document 8: Japanese Unexamined Patent Application Publication No. 2004-6271    Patent Document 9: International Patent Publication WO 2004/085360 Pamphlet    Patent Document 10: International Patent Publication WO 2004/085359 Pamphlet    Patent Document 11: Japanese Unexamined Patent Application Publication No. 2004-311182